AB-15 /MS 116-117-118 antenna fail

[ramcatdoc]

I have a CB radio on my CUCV with an AB15 base and the MS116-117-118 antenna. The antenna base is new/old stock fresh out of the box. I have cut my whip to be a total of 110 inches long from the bottom of the base to the antenna tip. I am using a new Cobra radio. My antenna tests good when it is fully vertical, but I get an "antenna fail" message when I have it tied down. I'm sure my connections are good. My question is, does the whip have to be fully extended (vertical) for it to be able to transmit well? I notice I can transmit normally at the lower channel numbers but not the higher, when the antenna is tied down.

Anybody have any ideas?

 

[SCSG-G4]

Allan,

Get a VOM, some alligator clips and a length of good copper wire. Disconnect the antenna wire from the radio and hook a clip to the end extending it out to the antenna. Measure the resistance at the tip of the antenna in both the vertical and tied down positions. If there is any difference at all on the most sensitive meter position, the problem may be in the antenna base, even though NOS. Go through it and loosen, then re-tighten any connections, then test again with the VOM.

 

[emmado22]

Whats the SWR?

 

[tennmogger]

You have provided a great set of descriptions of your trouble!

Antenna characteristics change as it is tied down nearer the vehicle, primarily from added capacitance from coupling to the vehicle. That changes the load on the transmitter and results in the "antenna fail" message. (what model of CB does that...a nice feature!)

Since the antenna works better at lower numbers = lower frequency, the antenna is too long. Shorten the antenna until it works best at the middle channel numbers, while positioned how you want best performance.

You might consider adding an adjustable stub at the top of the antenna, like a brass threaded rod, that you can adjust easily. (solder a nut or rod coupler to the end of the cut-off MS section.)

Have fun,

Bob

I have a CB radio on my CUCV with an AB15 base and the MS116-117-118 antenna. The antenna base is new/old stock fresh out of the box. I have cut my whip to be a total of 110 inches long from the bottom of the base to the antenna tip. I am using a new Cobra radio. My antenna tests good when it is fully vertical, but I get an "antenna fail" message when I have it tied down. I'm sure my connections are good. My question is, does the whip have to be fully extended (vertical) for it to be able to transmit well? I notice I can transmit normally at the lower channel numbers but not the higher, when the antenna is tied down.

Anybody have any ideas?

 

[m38mike]

Antennas tend to transmit straight out from their mast. So if you bend the mast so it's parallel to the vehicle, then you're going to transmit into the vehicle body, and the reflectance may be what the radio is seeing. I think if you check SWR with the antenna tied down, then again with the antenna straight up, you'll see a difference. Bottom line is you can receive with the antenna tied down, but you'd best have it pointed straight up to transmit.

 

[hunderliggur]

However at the lower Amateur frequencies (and military) say 7mhz to 14 MHz, the folded antenna makes a Near Vertical Incidence Skywave antenna that radiates up and reflects down off the ionosphere. This gives a several hundred mile range that is difficult to geolocate from the ground.

I bet you have a break of some kind in the base or a short. I can see the SWR rising but not so much for antenna fail UNLESS your hold down is shorting the antenna. You should try an insulator between the antenna and hold down.

 

[ke6rwj]

i had the same thing one, the rubber base has wire in it, turned out to be broken, when you bent it, it disconnected...

 

[ramcatdoc]

antenna fail

Whats the SWR?

The SWRs are better when the antenna is up. However, when tied down, they approach 3.

 

[ramcatdoc]

UNLESS your hold down is shorting the antenna. You should try an insulator between the antenna and hold down.

I wondered about that, but the tie down cord is just paracord, and it came with the clip as military surplus. I have tried holding the antenna with my hand rather than the tie down, and it still fails. (not sure if my body hindered the antenna any)

 

[ramcatdoc]

i had the same thing one, the rubber base has wire in it, turned out to be broken, when you bent it, it disconnected...

I actually replaced the base with an NOS one. I know that doesn't mean it still can't have a break, but this thing is pristine, and the exact same thing is happening with the new base as with the old one. Makes me think this has to do more with physics than anything else.

 

[hunderliggur]

Your hand will definitely affect the antenna, not to mention the possibility for a nasty RF burn. Try contacting your local Amateur radio club and see if someone can help you with a MFJ antenna analyzer.

 

[M813A1]

Check the ground for the Body to the frame and the ground for the Antenna mount !

 

[ramcatdoc]

Your hand will definitely affect the antenna, not to mention the possibility for a nasty RF burn. Try contacting your local Amateur radio club and see if someone can help you with a MFJ antenna analyzer.

I am using CB, only 4 watts. Is there a risk for RF burns with CB, or is this more likely with amateur and military radios?

 

[M813A1]

I am having a similar issue with my setup and burned up my little Midland radio I had my radio hooked to 12 v and it still burned up !! I have the same setup I check the continuity on all pieces before putting it together check the power and it still burned up ?? So I have gone back through and applied new grounds to the mount base , cargo body and cab to frame and cleaned the ground to the Batteries to frame too now just have to get a new radio

 

[rickf]

You do not want to be transmitting with a 3:1 match. You could fry the finals in the radio real quick.

 

[M813A1]

Heck I did not even transmitt it just smoked for some reason I think I am getting 24v when I have it hooked as 12v on the battery !! Not sure why this is happening ?? I am thinking it is the Hawker Gel Batteries that may be causing the problem and burned out my radio ? might be to high of amps for the radio ??

 

[jaxsof]

I was thinking you might have a broken wire in (the center of) the spring ( Ive seen more than a few of those), but you said you changed out the base.

Even at 4 watts, grabbing hold of an antenna isnt a habit you want to get into. Several years ago an aircraft carrier was at sea, and the Marines had a detachment painting the hull. One of the men went to reposition himself by grabbing hold of an antenna, and they were short one Marine.

Moral of the story is, if it may be a hot antenna, dont touch it. If the amp in the CB had a transistor going away, it may have hurt a little bit.
enough of the

Let us know what you find.

 

[ramcatdoc]

duplicate post

 

[ramcatdoc]

Even at 4 watts, grabbing hold of an antenna isnt a habit you want to get into. Several years ago an aircraft carrier was at sea, and the Marines had a detachment painting the hull. One of the men went to reposition himself by grabbing hold of an antenna, and they were short one Marine.

Yikes! Sort of like assuming all guns are loaded until you prove it's not yourself! I'll start that habit now. Thanks for the heads up!

 

[ramcatdoc]

Thanks to all of you for your advice. I think what I'll do is use my VOM to check for continuity with the antenna straight and bent. I'll disconnect it from the antenna wire (to avoid RF burns and to isolate the antenna/base).

Y'all have mentioned checking the ground connections in the truck. Help me with that. I know the battery (batteries?) is (are?) supposed to be grounded to the truck frame. Are there any other grounding connections in the CUCV itself? Where would i typically find them? How could I improve them if they are intact?

My AB-15 is grounded via the grounding plate to the antenna wire coaxial connector, as per our conversations in a previous thread. I have also, as per the same thread, added an additional grounding wire from the base directly to the mounting bracket attached to the truck. I know this is a good ground.

I'll try these things and hopefully will have a good result to report.

Any other thoughts?

Thanks y'all.